Python 1-1

Input, output, variables, types, conditionals

Input => Process => Output

1. Identify the problem inputs (requirements)

2. Identify the problem outputs (results)

3. Write an algorithm to transform inputs to outputs.

4. If you don’t know how to do a step… research it!

Python input and output

print() does output
input() does input, returns input so you must assign it to a variable

x = input("Enter something: ")
print(x)

Enter something:  jdshak
jdshak

Code Challenge 1-1-1

Write a program to input your first name and last name then output your last name, first name

F-Strings

• F-Strings are Python’s answer to string interpolation.

• This replaces the variable name with its value within a string.

• Called an F-string because the f tells Python to interpolate the string.

name = 'George'
print("{name} was curious.")
print(f"{name} was curious.")

Check Yourself 1

Which is an example of a properly used string literal?

A. print(welcome)
B. print("welcome")
C. print "welcome"
D. print welcome

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Variables

• Variables are named areas of computer memory for storing data.

• The name can be anything but should make symbolic sense to the programmer.

• We write to the variable’s memory location with the assignment statement (=)

• We read from the variable by calling its name.

• Variable names must begin with a letter or _ and must only contain letters, numbers or _.

Variables are of a Specific Type

Type	Purpose	Examples
int	Numeric type for integers only	45, -10
float	Numeric type floating point numbers	45, -10
bool	True or False values	True, False
str	Characters and text	"A", 'Mike'

Type Detection and Conversion

Python Function	What It Does	Example of Use
type(n)	Returns the current type of n	type(13) == int
int(n)	Converts n to type int	int("45") == 45
float(n)	Converts n to type float	float(45) == 45.0
str(n)	Converts n to type str	str(4.0) == '4.0'

Programmatic Expressions

Programmatic Expressions contain operators and operands. They evaluate to a value, preserving type:

print(2 + 2)
print(2.0 + 2)
print("sh" + 'ip')
print('hi' + 2) # error

4
4.0
ship

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[4], line 4
      2 print(2.0 + 2)
      3 print("sh" + 'ip')
----> 4 print('hi' + 2)

TypeError: can only concatenate str (not "int") to str

Arithmetic Operators

Operator	What it Does	Example of Use
+	Addition or string concenation	3 + 4 == 7
-	Subtraction	4 - 3 == 1
*	Multiplication	3 * 4 == 12
/	Division	4 / 3 == 1.33333
//	Intger division (quotent)	13 // 3 == 4
%	Modulo (remainder)	13 % 3 == 1
( )	Force an order of operations	2 * (3 + 4) == 14

Code Challenge 1-1-2

Let’ write a program to divide up the check among diners in a party.

Write a program to input the amount of a restaurant check, tip %, and number of diners

The program should output the total amount with tip, and the amount each diner owes.

Check Yourself: Which Type 1?

What is the value of str(314) ?

A. 314
B. "314"
C. int
D. '34.0'

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Check Yourself: Which Type 2?

What is the value of type(314.0) ?

A. 314
B. float
C. int
D. '314.0'

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Check Yourself: Operators

What is the output of the following python code?

a = 10
b = 2
c = 1 + (a/b)
print(c)

A. 6
B. 5.5
C. 6.0
D. 5

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Program Flow Control with IF

• The IF statement is used to branch your code based on a Boolean expression.

if boolean-expression:
    statements-when-true
else:
    statemrnts-when-false

Python’s Relational Operators

Operator	What it does	Examples
>	Greater than	4>2 (True)
<	Less than	4<2 (False)
==	Equal To	4==2 (False)
!=	Not Equal To	4!=2 (True)
>=	Greater Than or Equal To	4>=2 (True)
<=	Less Than or Equal To	4<=2 (True)

Expressions consisting of relational operators evaluate to a Boolean value

Code Challenge 1-1-3

a presssure sensor determines whether or not to open a door

When the pressure is larger than 10 we want the door open

otherwise we want it closed

write code to simulate this

Check Yourself: Relational Operators

On Which line number is the Boolean expression True?

x = 15
y = 20
z = 2
x > y
z*x <= y
y >= x-z
z*10 == x

A. 4
B. 5
C. 6
D. 7

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Python’s Logical Operators

Operator	What it does	Examples
and	True only when both are True	4>2 and 4<5 (True)
or	False only when both are False	4<2 or 4==4 (True)
not	Negation(Opposite)	not 4==2 (True)
in	Set operator	4 in [2,4,7] (True)

Check Yourself: Logical Operators

On Which line number is the Boolean expression True?

raining = False
snowing = True
age = 45
age < 18 and raining
age >= 18 and not snowing
not snowing or not raining
age == 45 and not snowing

A. 4
B. 5
C. 6
D. 7

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Multiple Decisions: IF ladder

Use elif to make more than one decision in your if statement. Only one code block within the ladder is executed.

if boolean-expression1:
    statements-when-exp1-true
elif boolean-expression2:
    statements-when-exp2-true
elif boolean-expression3:
    statements-when-exp3-true
else:
    statements-none-are-true

#Elif versus multiple ifs...
# One decision or multiple decisions. 

x = int(input("enter an integer"))

# one decision
if x>10:
    print("A:bigger than 10")
elif x>20:
    print("A:bigger than 20")    

    # Multiple decisions
if x>10:
    print("B:bigger than 10")
if x>20:
    print("B:bigger than 20")

enter an integer 50
A:bigger than 10
B:bigger than 10
B:bigger than 20

Check Yourself: IF Statement

Assuming values x = 25 and y = 6 what value is printed?

if x > 20:
    if y == 4:
        print("One")
    elif y > 4:
        print("Two")
    else:
        print("Three")
else:
    print("Four")

A. One
B. Two
C. Three
D. Four

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Code Challenge 1-1-4

Number to Letter grade

• Letter grades in a college class are computed as follows:

  • 95 and above is an A

  • 75 and above, but below 95 is a B

  • 50 and above, but below 75 is a C

  • below 50 is F

• Write a program to input the number grade and calculate the letter grade

• Re-write to account for “bad” grades > 120 or < 0